Abstract: In order to protect drinking water systems and aquatic ecosystems, we need to identify critical nutrient source areas and transport mechanisms. The primary transport mechanism for phosphorus and other reactive contaminants has been considered to be surface runoff with subsurface transport considered to be negligible. However, local or regional conditions can result in subsurface transport becoming important. Researchers currently do not understand the potential significance of connectivity between phosphorus in surface runoff and groundwater and phosphorus movement from the soil to groundwater in watersheds with cherty and gravelly soils. The proposed research hypothesizes that macropores and gravel outcrops in alluvial floodplains have a significant, scale-dependent impact on contaminant leaching through soils; therefore, both soil matrix and macropore infiltration must be accounted for in nutrient transport analyses in riparian floodplains associated with gravel bed streams. However, quantifying the occurrence and spatial variability of macropores and gravel outcrops in the subsurface of alluvial floodplains is difficult without innovative field studies. This research proposes an integrated field, laboratory and numerical modeling project that includes geophysical imaging, characterization of phosphorus mobility and transport, and tracer and phosphorus injection experiments.

Transformative Hypotheses/Research Advances: The potential for phosphorus leaching is commonly estimated based on point-measurements of soil test phosphorus (STP) or measurements of the sorption capability of disturbed soil samples representing the soil matrix. However, macropores and gravel outcrops, which frequently occur in Ozark alluvial floodplains of eastern Oklahoma and western Arkansas, are hypothesized to have a significant impact on water and contaminant movement to ground water and on the interaction between stream and floodplain ground water systems. The impact of experimental scale on phosphorus leaching in alluvial floodplains in the Ozark Ecoregion will be evaluated to answer the question of what minimum land area is necessary to adequately quantify phosphorus leaching. This research hypothesizes that phosphorus leaching will generally increase as the scale increases from point to plot scales due to heterogeneity within these floodplain systems. This will be evaluated by measuring phosphorus leaching at the point scale in the laboratory using flow cell experiments and at plot scales (1, 10, and 100 square meters) with replicated infiltration experiments at three riparian floodplain sites in Oklahoma and Arkansas. It is hypothesized that variability in small scale measurements will converge as larger plot scales approach a representative volume element. Through numerical modeling, the research will be extended beyond the three specific floodplain sites by estimating the phosphorus concentration and load entering gravel subsoils for various topsoil depths, storm sizes, and initial phosphorus concentrations for the Ozark Ecoregion.

Broader Impacts: This research will have wide reaching implications for management of riparian floodplains in alluvial systems worldwide and the implications will extend beyond phosphorus to other conservative and reactive solutes such as pesticides, pathogens, and other emerging contaminants. Hundreds of millions of dollars are spent each year to mitigate surface runoff, sediment and nutrient loads. Although these management plans can be effective, this research hypothesizes that subsurface contaminant transport could also be a contributing factor in certain conditions. If subsurface transport of phosphorus and other contaminants to alluvial groundwater is significant, current floodplain conservation practices will not be as effective. Therefore, it will be necessary to consider these mechanisms when identifying and developing appropriate and effective conservation practices. The PI, co-PIs, and graduate and undergraduate students will disseminate the scientific information through peer-reviewed publications, fact sheets, appearance on an Oklahoma TV program, and presentations at local, state, national, and international meetings.